[Java] Fix splitAndTransfer for variable width vector

When we splitAndTransfer a vector, the value count to set for the target vector should be equal to split length and not the value count of source vector. 

We have seen cases in operator slike FLATTEN and under low memory conditions, we end up allocating a lot more memory for the target vector because of using a large value in setValueCount after split and transfer is done.

**Reporter**: [Siddharth Teotia](https://issues.apache.org/jira/browse/ARROW-2294) / @siddharthteotia

_{**Note**: *This issue was originally created as [ARROW-2294](https://issues.apache.org/jira/browse/ARROW-2294). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

在 Java 实现中搜索 splitAndTransfer 和可变宽度向量类，然后跟踪 FLATTEN 如何调用该操作。验证传输后目标向量的值数量是否与 split 长度一致，并检查相关的现有测试，或为减少分配的情况补充覆盖率。

由索引模型根据 Issue 内容生成。